Miniaturization of computers has proceeded from desk-top size computers through lap-top and note-book sizes and even to pocketbook sizes, recently. With such miniaturization, commercially available magnetic disk devices have been miniaturized from 3.5 inches, through 2.5 inches and 1.8 inches and even to 1.3 inches. This tendency is also true for optical disk devices. In the following description, the term "disk device" incudes both magnetic disk devices and optical disk devices.
With the miniaturization of the disk device and devices to be mounted thereon, it has been desired to reduce the power consumption thereof. An example of such prior art techniques for reducing the power consumption of a magnetic disk device is disclosed in U.S. Pat. No. 4,933,785 (corresponding to Japanese Patent Application Publication No. 3-503101). In this technique, the operation mode is divided into a sleep mode in which circuit functions other than a minimum interface function for an up-link connection necessary for restart of the CPU are stopped; an idle mode in which the CPU is operative and other functions of the spindle motor circuit, a servo circuit and a recording and reproducing circuit, etc., other than the up-link connection function are stopped; and another idle mode in which the servo function for a rotating spindle motor is operative and the data recording and reproducing circuit as well as the usual write and read seek mode are inoperative. Accordingly, there are specific power supply requirements for the respective modes, and the power supplied to circuit portions which are not needed to function in a specific mode is reduced. In this method, however, there is no provision made for the reduction of the power source current required to be provided for the start time of a disk device including during the initial portion of a start period of a spindle motor from a state wherein the disk is not spinning.
In another example, there is a method disclosed in Japanese Patent Application laid-open No. H4-205963, wherein power consumption during re-start of a spindle motor is reduced by reducing the rotation speed of the spindle to an extent that its rotation can be maintained in an idle mode in which write and read is not performed. Although this method is effective to reduce power consumption without increasing the required time for re-start, there is still no provision made for sufficiently reducing the power source current required during the start time of the disk device spin up during the initial portion of the start period for spin up of a spindle motor powered disk device from a motionless state of the disk.
On the other hand, in consideration of providing an alternative approach to meeting the low power and high speed start up requirements for miniaturized devices, initial start software, such as an OS (operating system) of a computer, has been expanded and diversified as can be seen in, for example, UNIX, so that a memory device can be provided that has large memory capacity, which is required for storing such software information. Although semiconductor memory devices are suitable for large capacity storage and short initial start times, a magnetic disk device is still preferred since there is a high cost per memory capacity of such semiconductor memories, compared with that of magnetic disk devices.